REACTIVITY OF AROMATIC RINGS

In each of the five sections of Chapter 3, the chemistry is reviewed in the following order (1) Reactivity of aromatic rings (thermal reactions not involving reagents, substitutions at carbon, additions to nitrogen, metallations) (2) Reactions of nonaromatic compounds (this enormous area, which overlaps extensively with nonheterocyclic chemistry, is reviewed with emphasis on the heterocyclic aspects) (3) Reactions of substituents (with emphasis on situations in which substituents behave somewhat differently when attached to a heterocycle note that for benzene-fused heterocycles, the benzene ring is treated as a substituent). 

Like ammonia, the throe classes of amines contain nitrogen that bears an unshared pair of electrons as a result, amines closely resemble anrmonia in chemical properties. The tendency of nitrogen to share this pair of electrons underlies the entire chemical behavior of amines their basicity, their action as nucleophiles, and the unusually high reactivity of aromatic rings bearing amino or substituted amino groups. 

Increasing reactivity of aromatic ring to electrophilic attack... 

Even without lithium, the reactivity of aromatic rings is nucleophilic. But the source of that nu-cleophilicity is very different when a lithium atom is in place. The lithium atom does more than dramatically increase the reactivity of the benzene ring, it alters its character entirely. 

The effect of halogens on the reactivity of aromatic rings is described on pp. 489-490 of the textbook. 

Arylamines contain two functional groups the amine group and the aromatic ring they are difunctional compounds The reactivity of the amine group is affected by its aryl substituent and the reactivity of the ring is affected by its amine substituent The same electron delocalization that reduces the basicity and the nucleophilicity of an arylamme nitrogen increases the electron density in the aromatic ring and makes arylamines extremely reactive toward electrophilic aromatic substitution... 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

PMDI is produced on an industrial scale by the phosgenation of diamin-odiphenylmethane. Structure and molar mass of PMDI depend on the number of aromatic rings in the molecule. For PMDI the distribution of the three monomeric isomers has a great influence on the quality, because the reactivities of the various isomers (4,4 -, 2,4 - and 2,2 -MDI) differ significantly. The greater the portion of the 2,2 - and 2,4 -isomers, the lower is the reactivity. This can lead to different bonding strengths as well as to residual isomers in the produced wood-based panels. 

The effects of the nucleophile on aromatic substitution which are pertinent to our main theme of relative reactivity of azine rings and of ring-positions are brought together here. The influence of a nucleophile on relative positional reactivity can arise from its characteristics alone or from its interaction with the ring or with ring-substituents. The effect of different nucleophiles on the rates of reaction of a single substrate has been discussed in terms of polarizability, basicity, alpha effect (lone-pair on the atom adjacent to the nucleophilic atom), and solvation in several reviews and papers. ... 

Blackley548 measured the rates of deuteration of biphenylene, fluorene, tri-phenylene, and phenanthrene relative to o-xylene as 6.15 5.85 1.08 1.32, which is in very good agreement with the values of 8.80 7.00 - 1.14 which may be deduced from the detritiation data in Table 159, obtained using anhydrous trifluoroacetic acid. Aqueous trifluoroacetic acid (with the addition in some cases of benzene to assist solubility) was used by Rice550, who found that triptycene was 0.1 times as reactive per aromatic ring as o-xylene (cf. 0.13 derivable from Table 159) whereas the compound (XXXI) was 0.9 times as reactive as o-xylene. An exactly comparable measure is not available from Table 158, but dihydroanthracene (XXXII), which is similar, was 0.51 times as reactive as o-xylene and... 

Carbenes are so reactive that they add to the double bonds of aromatic rings. The products are usually not stable and rearrange to give ring expansion. Carbene reacts with benzene to give cycloheptatriene ... 

The Friedel-Crafts alkylation reaction does not proceed successfully with aromatic reactants having EWG substituents. Another limitation is that each alkyl group that is introduced increases the reactivity of the ring toward further substitution, so polyalkylation can be a problem. Polyalkylation can be minimized by using the aromatic... 

The outer rings of the anthraquinone molecule (52) are aromatic in nature and as such are capable of undergoing substitution reactions. The reactivity of the rings towards substitution is determined by the fact that... 

Inspired by the results of aromatic-ring hydroxylation from the laboratory of Karlin and co-workers, a few groups provided further examples of such reactivity, including some structurally characterized complexes of modified m-xylyl-based pyridine-donor ligands (Schiff base and non-Schiff base acyclic ligands), as well as aliphatic amine donor ligands (179) (Cu-Cu 2.990 A),169 (180) (Cu-Cu 3.015 A),170 and (181) (Cu-Cu 2.999 A).171 172 A m-xylyl-based ligand system that was used by Mukherjee and co-workers in the formation of complex (181) also resulted in the isolation of a bis(/i-hydroxo)dicopper(II) complex (182) (Cu-Cu 3.004 A).171,172 Casella and co-workers demonstrated that when their dicopper(I) complex... 

Reactivity of Nonconjugated Rings 5.08.6.1 Isomers of Aromatic Compounds... 

LiP catalyze several oxidations in the side chains of lignin and related compounds [26] by one-electron abstraction to form reactive radicals [27]. Also the cleavage of aromatic ring structures has been reported [28]. The role of LiP in ligninolysis could be the further transformation of lignin fragments, which are initially released by MnP. 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

The molecular orbital picture of benzene proposes that the six jt electrons are no longer associated with particular bonds, but are effectively delocalized over the whole molecule, spread out via orbitals that span all six carbons. This picture allows us to appreciate the enhanced stability of an aromatic ring, and also, in due course, to understand the reactivity of aromatic systems. There is an alternative approach based on Lewis structures that is also of particular value in helping us to understand chemical behaviour. Because this method is simple and easy to apply, it is an approach we shall use frequently. This approach is based on what we term resonance structures. 

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